The aforementioned copending application discloses an improved apparatus and method for preheating solid aggregate material prior to introduction into a rotary kiln by contacting the incoming aggregate with the waste heated gases which are discharged from the kiln. In accordance with the apparatus and method disclosed therein, the aggregate is directed downwardly along a predetermined path of travel while being maintained in the form of a relatively thin layer and while the heated gases which are discharged from the kiln are directed upwardly along a predetermined sinuous path of travel repeatedly passing back and forth through the downwardly moving thin layer of aggregate from the opposite sides thereof to thus provide highly effective contact of the gas with the aggregate. The thin layer of aggregate is guided downwardly along a passageway formed by a pair of gas permeable retaining walls which extend generally vertically in opposing relation to one another. The retaining walls are of a nonlinear zigzag configuration so arranged as to direct the thin layer of aggregate along a sinuous or zigzag path of travel in the course of its downward movement along the elongate passageway.